1. Field
This invention relates generally to a fiber laser amplifier having higher power and narrower linewidth and, more particularly, to a fiber laser amplifier that includes a harmonic phase modulation device that applies an RF frequency modulation (FM) signal to a seed beam to remove optical power from a zeroth-order frequency and create harmonic sidebands of the beam, where the modulated beam is temporally dispersed before being amplified by a non-linear amplifier.
2. Discussion
High power laser amplifiers have many applications, including industrial, commercial, military, etc. Designers of laser amplifiers are continuously investigating ways to increase the power of the laser amplifier for these and other applications. One known type of laser amplifier is a fiber laser amplifier that employs a doped fiber that receives a seed beam and a pump beam to amplify the seed beam and generate the laser beam, where the fiber has an active core diameter of about 10-20 μm or larger.
Improvements in fiber laser amplifier designs have increased the output power of the fiber to approach its practical power and beam quality limit. To further increase the output power of a fiber amplifier some fiber laser systems employ multiple fiber laser amplifiers that combine the amplified beams in some fashion to generate higher powers. A design challenge for fiber laser amplifier systems of this type is to combine the beams from a plurality of fiber amplifiers in a manner so that the beams provide a single beam output having a uniform phase over the beam diameter such that the beam can be focused to a small focal spot. Focusing the combined beam to a small spot at a long distance (far-field) defines the quality of the beam.
In one known multiple fiber amplifier design, a master oscillator (MO) generates a seed beam that is split into a plurality of fiber seed beams each having a common wavelength, where each fiber beam is amplified. The amplified fiber seed beams are then collimated and directed to a diffractive optical element (DOE) that combines the coherent fiber beams into a single output beam. The DOE has a periodic structure formed into the element so that when the individual fiber beams each having a slightly different angular direction are redirected by the periodic structure all of the beams diffract from the DOE in the same direction. Each fiber beam is provided to a phase modulator that controls the phase of the beam so that the phase of all the fiber beams is maintained coherent. However, limitations on bandwidth and phasing errors limits the number of fiber beams that can be coherently combined, thus limiting the output power of the laser.
In another known multiple fiber amplifier design, a plurality of master oscillators (MOs) generate a plurality of fiber seed beams at a plurality of wavelengths, where each fiber beam is amplified. The amplified fiber seed beams are then collimated and directed to a diffraction grating, or other wavelength-selective element, that combines the different wavelength fiber beams into a single output beam. The diffraction grating has a periodic structure formed into the element so that when the individual fiber beams each having a slightly different wavelength and angular direction are redirected by the periodic structure all of the beams diffract from the diffraction grating in the same direction. However, limitations on bandwidth limit the number of fiber beams that can be wavelength-combined, thus limiting the output power of the laser.
To overcome these limitations and further increase the laser beam power, multiple master oscillators are provided to generate seed beams at different wavelengths, where each of the individual wavelength seed beams is split into a number of fiber seed beams and where each group of fiber beams has the same wavelength and are mutually coherent. Each group of the coherent fiber seed beams at a respective wavelength are first coherently combined by a DOE, and then each group of coherently combined beams are directed to a spectral beam combination (SBC) grating at slightly different angles that diffracts the beams in the same direction as a single combined beam of multiple wavelengths. The SBC grating also includes a periodic structure for combining the beams at the different wavelengths.
It is often desirable that the output beam from a fiber amplifier be narrow linewidth, i.e., have a narrow frequency range, to improve beam quality. However, providing both high power and narrow linewidth have heretofore been challenging in the art because those requirements are typically incompatible with each other because higher power typically requires a wider beam linewidth. More particularly, the phenomenon of stimulated Brillouin scattering (SBS) i.e., back scattering of the beam as it propagates along the fiber amplifier, increases at narrower linewidths with small frequency ranges, which acts to reduce beam power. However, the wider the beam linewidth, the more difficult it is to coherently combine or spectrally combine beams from multiple fibers into a single beam through known beam combining techniques. Particularly, dispersion effects from an SBC grating require that the linewidth of the beams being amplified is narrow, where spectral dispersion causes the spectral components of the beam to be diffracted at different angles. In other words, for SBC, the spectral brightness of the seed beam directly limits the theoretical brightness of the combined beam output.
For coherent beam combining (CBC), the spectral brightness of the seed beam limits the combining efficiency owing to imperfect matching of group delay and dispersion between amplifiers. Typically, the source spectral brightness is limited by SBS, and the seed beam source to the fiber amplifier must be frequency-modulated to reduce the peak SBS gain and achieve the desired output power. The FM spectral broadening limits the attainable spectral brightness from a single fiber amplifier, thus limiting the system output.
In order to overcome these limitations in the known fiber laser amplifier systems, designers of fiber amplifiers typically employ one or more phase modulators before the amplification stage in the fiber amplifier to reduce the linewidth via frequency modulation. However, once the modulation is applied to the beam before it is amplified, that widening of the spectral content of the beam is carried through the amplifier resulting in a low spectral brightness amplified beam. Hence, there is a need in the art for fiber amplifiers having a higher spectral brightness that is currently obtainable by FM broadening.